Lean-burn engines that are fuelled with a gaseous fuel suffer from unburned hydrocarbon emissions, especially at low engine loads when the equivalence ratio and cylinder temperature are too low to fully oxidize the fuel. Methane emissions are particularly problematic since methane (a greenhouse gas) cannot be oxidized in an oxidation catalyst at typical exhaust temperatures in lean-burn engines. Lean-burn engines are defined herein to be engines that operate with an equivalence ratio less than 1.0 for at least a majority of engine operating conditions and typically over the full range of engine operating conditions. Examples of lean-burn engines include high pressure direct injection (HPDI) engines that are mainly fuelled with natural gas and employ a pilot fuel, such as diesel, to ignite the main fuel, non-premixed engines that employ other ignition devices (such as a hot surface, a hot element, or a spark plug) and lean-burn fumigated engines that employ either a pilot fuel or an ignition device such as a spark plug as an ignition mechanism.
Lean-burn engines control engine load by varying fuelling quantity without necessarily changing the air system, and as a result the equivalence ratio (EQR) can vary over the range of engine operating conditions. This is unlike stoichiometric engines that maintain an equivalence ratio of one (1) over the majority of engine operating conditions. In a stoichiometric engine both the air system and the fuelling quantity are adjusted for each commanded engine load. For example, the air system can be adjusted by a throttle and as the mass air flow changes the fuelling quantity is simultaneously adjusted to achieve the desired load. Careful control is needed to ensure that the fuel-air equivalence ratio does not deviate from the stoichiometric ideal. For lean-burn engines the calibrated equivalence ratio at any one engine-load/engine-speed combination is a balance between competing demands, such as between combustion performance and emissions, maintaining smooth transitions during transient engine operating conditions (“map smoothness”), and avoiding compressor surge. This typically results in using low equivalence ratios (less than 0.4) at loads below 25% of full engine load, and under these circumstances methane emissions are relatively high.
Unburned hydrocarbon emissions can occur when gaseous fuel is over-mixed with air resulting in excessively low local equivalence ratios in certain regions of the combustion chamber. In these regions the gaseous fuel and air mixture is near or below the lower flammability limit and is too lean to burn. Other sources of unburned hydrocarbons include crevice and quench regions, where the flame cools too rapidly and is hence extinguished, or in rich areas of the reaction that never properly mix with an oxidizer, and hence are never fully burned. By increasing the temperature in the combustion chamber, the amount of fuel in the lean regions that can react increases; this leads to lower emissions of unburned gas. As a result, most previous techniques to control unburned emissions have focused on increasing in-cylinder temperatures. Techniques that lead to lower in-cylinder temperatures would generally be expected to make unburned hydrocarbon emissions worse for lean-burn engines.
Previous attempts at reducing unburned hydrocarbon emissions included combustion chamber mixing techniques that reduce the chance of forming regions where the local equivalence ratio was near or below the lower flammability limit. With manifold or port fuel injection the fuel will be largely premixed before combustion but the burning rate can be significantly enhanced by port and combustion chamber design to augment turbulence. With direct (late cycle) fuel injection the dominant turbulence source will be the jet injection momentum. Thus load (as well as injection timing) can significantly affect burning rate. In general with late injection it may be difficult to achieve satisfactory mixing and burning at low load even if the equivalence ratio is so low as to provide abundance of oxygen.
The state of the art is lacking in techniques for reducing unburned hydrocarbon emissions in lean-burn engines that are fuelled with a gaseous fuel. The present method and apparatus provide a technique for improving unburned hydrocarbon emissions in gaseous fuelled lean-burn internal combustion engines.